Oxygen is used in a wide variety of processes in industry and is usually generated either by cryogenic distillation of air or by adsorptive separation of air using zeolites in a vacuum or pressure swing mode. For medical, metal cutting and small scale cylinder filling, the purity of the oxygen must be greater than 95 volume % oxygen. This level of purity has excluded traditional adsorptive separations using traditional 5A and X zeolites which adsorb nitrogen while passing oxygen and argon. Such separation processes limit oxygen concentration to approximately 95 volume % from an air or enriched air feed. Thus such markets have to be served by liquid oxygen either in Dewars or tanker trucks or by cylinders.
Pressure and vacuum swing adsorption processes have been one of the traditional methods utilized for separation of air into components to meet smaller scale production requirements. Crystalline zeolitic molecular sieves have been widely utilized in these processes taking advantage of their differential selectivity with respect to the gaseous components. A considerable technology base has been developed to alter the differential selectivity of these zeolites. For example, the cage structure of the crystalline zeolitic molecular sieves has been altered to permit selective adsorption of gases. In addition, the silicon-aluminum ratio and the type of cations present in the crystalline zeolite can affect the adsorption characteristics. Both of these properties have been modified in an effort to tailor the number of cation sites and charge characteristics of the zeolites and thereby alter the adsorption characteristics. The following references are cited to show various zeolite compositions for use in pressure and vacuum swing adsorption systems relevant to the current invention.
Wilkerson, B. E., xe2x80x9cThe Adsorption of Argon and Oxygen on Silver Mordenitexe2x80x9d, Master""s Thesis, The Ohio State University (1990) discloses the use of silver mordenite as a selective adsorbent for the separation of oxygen from a feedstream of 95 volume % oxygen and 5 volume % argon. Mordenites of different silver concentrations were prepared and equilibrium isotherms of argon and oxygen at various temperatures were measured. The experimental data show that a sodium mordenite shows no selectivity between argon and oxygen, but a highly concentrated silver mordenite shows selectivity for separating oxygen from argon.
U.S. Pat. No. 5,226,933 teaches a process for making  greater than 95 volume % purity oxygen from a 95 volume % oxygen and 5 volume % argon feed by the use of silver mordenite as an adsorbent.
U.S. Pat. No. 5,470,378 discloses a process for separating argon from a feed gas comprising oxygen and argon to yield high purity oxygen at pressures between 5 and 160 psia utilizing an X zeolite where at least 80% of the available sites are occupied by silver. In the process, at least a portion of the argon in the feed gas is adsorbed by the AgX bed, thereby leaving an oxygen-enriched gas stream. The AgX zeolites are formed by ion exchanging NaX zeolite with a silver salt, such as silver nitrate, although other types of zeolites, e.g., CaX zeolite, may be used. Product gas streams of 99 volume % oxygen and less than 1 volume % argon are reported.
Application 10-152305 (1996), xe2x80x9cOxygen Gas Production Equipment and Oxygen Gas Production Methodxe2x80x9d, Teruji, K. This article discloses the use of a silver-exchanged sodium or calcium X zeolite for producing high purity oxygen. The silver-exchanged zeolites are formed by contacting a sodium- or calcium-based X zeolite with a silver salt to a preselected silver exchange level, e.g., 10 to 100%. Argon/oxygen selectivities are achieved only at the high level of silver exchange, e.g., 90%.
Yang, R. T., Chen, Y. D., Peck, J. D., and Chen, N., xe2x80x9cZeolites Containing Mixed Cations for Air Separation by Weak Chemisorption-Assisted Adsorptionxe2x80x9d, Ind. Eng. Chem. Res., 35, pp. 3093-3099 (1996) compared the nitrogen and oxygen adsorption isotherms for approximately 85% Li, 15% NaX, xcx9c100% AgX, and 20% AgLiNaX (called LiAgX in the reference) samples. The starting zeolite for preparing these samples was NaX (13X). Sodium X zeolites were ion exchanged to obtain lithium X and silver X zeolites as well as a mixed lithium and silver X zeolite. The lithium/silver zeolites were sequentially exchanged by first fully exchanging the sodium cations of the X zeolites with lithium cations followed by exchange of a portion of the lithium cations with silver cations to a level of approximately 20%. It was concluded that AgX zeolite is undesirable for air separation due to its N2/O2 selectivity at low pressures. The authors observed that the LiAgX sample had a higher N2/O2 selectivity than LiX above total pressure of 0.07 atm and a lower selectivity than LiX at lower total pressures. The lower selectivity at lower pressures was asserted to aid in removal of nitrogen during the regeneration part of a process cycle. Combined with a higher N2 capacity for the LiAgX, the authors concluded that LiAgX was superior to LiX for air separation, under proper vacuum swing conditions.
Hutson, N. D., Rege, S. U., and Yang, R. T., xe2x80x9cMixed Cation Zeolites: LixAgy-X as a Superior Adsorbent for Air Separationxe2x80x9d, AlChE Journal, 45(4), pp.724-734 (1999) teaches a way to improve the air separation performance of LiX type zeolites. By adding a very small amount of Ag to LiX zeolites and subjecting the resulting zeolite to proper dehydration conditions, silver clusters are formed. These silver clusters enhance the nitrogen isotherm relative to the nitrogen isotherm for LiX. Best conditions for the formation of Ag clusters were reported to be drying the Ag-containing zeolites at room temperature, followed by dehydration in vacuum at a temperature of at least 450xc2x0 C., but no greater than 500xc2x0 C. for a minimum of 4 hours. Oxygen and argon isotherms were provided for LiLSX (low silica X) and AgLSX. The LiLSX had an argon/oxygen selectivity of  less than 1.0 (where the selectivity is the ratio of the slope of the isotherms at low pure gas loading) and the AgLSX had an argon/oxygen selectivity of xcx9c1.0. Additional zeolite compositions from 1.1 to 21 silver atoms per unit cell, with the balance being primarily lithium, were made. Nitrogen and oxygen isotherms were measured on these materials. An AgX sample was also prepared to compare nitrogen isotherms with the AgLSX samples.
Yang, R. T. and N. D. Hutson, xe2x80x9cLithium-Based Zeolites Containing Silver and Copper and Use Thereof for Selective Adsorptionxe2x80x9d, International Application, Publication WO 00/40332 presents the same samples which were discussed in Hutson et al. 1999 above. Low silica X-type zeolites (LSX) having an Si/Al ratio of 1.0 which have been subjected to appropriate cation exchange are used in the adsorption process. Several types of lithium/silver exchanged sodium X zeolites had been prepared wherein the silver exchange levels are 0.0, 1.1, 3.5, 11.5, and 21.0 atoms per unit cell out of 96 total. Two high silver sodium X zeolites, the first being AgNaLSX (95.7 silver atoms, 0.3 sodium atoms, 96 atoms/unit cell available) and the second being AgNaX (85.7 silver atoms, 0.3 sodium atoms, 86 atoms/unit cell available) were used for comparison purposes.
The silver exchanged sodium zeolites were reported as having high selectivity for nitrogen, but not preferentially selective for oxygen as compared to argon. It was suggested that the lithium/silver zeolite was most favorable for oxygen production.
Hutson, N. D. and Yang, R. T., xe2x80x9cStructural Effects on Adsorption of Atmospheric Gases in Mixed Li,Ag-X-Zeolitexe2x80x9d, AlChE Journal, 46(11), pp. 2305-2317 (2000) presents a study based on AgLiX to determine location of Ag clusters for various activation conditions. The isotherms presented were those presented earlier (Hutson et al. 1999). In addition to the compositions disclosed in WO 00/40332, this reference discloses two additional compositions used in the structural studies; these are 2.0 atoms Ag, 0.7 atoms Na, 93.3 atoms Li (96 atoms/unit cell) and 41.8 atoms Ag, 0.2 atoms Na, 54 atoms Li (96 atoms/unit cell).
Hutson, N. D., Reisner, B. A., Yang, R. T., and Toby, B. H., xe2x80x9cSilver Ion-Exchanged Zeolites Y, X, and Low-Silica X: Observations of Thermally Induced Cation/Cluster Migration and the Resulting Effects on the Equilibrium Adsorption of Nitrogenxe2x80x9d, Chem. Mater., 12, pp. 3020-3031 (2000) presents a study of highly silver exchanged Y, X, and LSX zeolites. Nitrogen isotherms are determined for samples activated under various atmospheres, temperatures, and durations. Correlation is made to Ag cluster formation and migration.
This invention relates to an improved crystalline zeolite material for use in effecting separation of argon from oxygen. The crystalline zeolitic material is an X-type zeolite having an argon/oxygen Henry""s Law constant ratio (also called selectivity) measured at 23xc2x0 C. greater than 1.05 and preferably greater than 1.15 and up to about 1.4 and at a silver exchange level of less than or equal to 70%. Unless otherwise noted, percent refers to percent of exchangeable cation sites in the zeolite. One method for preparing a silver exchanged X zeolite having a high argon/oxygen selectivity is to exchange the cations in an X zeolite first with lithium cations and then with silver cations to preselected levels. Proper ion exchange and calcination must be applied to the zeolite to achieve an argon/oxygen selectivity greater than 1. Preferably, a level of silver cation, based on total cation availability, from 20 to 70% is present in the X zeolite. More preferred levels of silver exchange in the lithium-exchanged X zeolite range from 30 to 60%. There are significant advantages associated with the crystalline zeolitic material described herein. These advantages include:
an ability to provide for enhanced argon/oxygen selectivity ratios at silver concentration of less than or equal to 70% exchange,
an ability to provide for efficient separation of argon from oxygen by use of a pressure or vacuum swing process,
an ability to provide high purity oxygen via a pressure or vacuum swing process, and,
lower cost.